Mode of gypsum deposition in southeastern Iranian soils as revealed by isotopic composition of crystallization water

Variations in δD and δ18O values of water of crystallization and meteoric water were used to elucidate the origin of mother brine and the gypsum mode of formation in selected southeastern Iranian soils. Thirteen representative pedons were sampled at various geomorphic positions around the city of Rafsanjan. Crystallization water in different parts of three gypsum pendants was also analyzed isotopically. The slope of the δD and δ18O plot for crystallization water was found to be 2.8, showing that evaporation was the major process for gypsum accumulation in the studied area. Comparison of data for eight rainwater samples and the crystallization water shows that gypsum formation occurred in a climate that was probably lower in temperature than that of today. The mean δD and δ18O values of gypsum crystallization water vary from −41.9‰ and +6.5‰ for rock pediment to −36.9‰ and +10.0‰ for mantled pediment and −28.8‰ and +13.8‰ for transitional surfaces and finally, −8.8‰ and +19.2‰ for playa margins. The existence of such a close relationship between the isotopic composition of gypsum crystallization water and geomorphic positions could be attributed to the deposition of gypsum in different landforms at various stages of evaporation and/or progressive evaporation of moving surface and subsurface waters. An increase in δD and δ18O values was also observed with increasing depth in almost all soil profiles studied. Different climates at the time of gypsum formation in different depths seem to be the possible reason. With increasing depth from the pebble surface in gypsum pendants, δD and δ18O values decrease, which is also attributed to climatic changes during pendant development.